The present invention relates generally to a rotating electrical machine employing a superconductive coil. More particularly, the present invention relates to a system and method for transferring a cryogenic fluid from a stationary source of cryogenic fluid to a rotating superconductive coil.
Rotating electrical machines, such as motors and generators, have a rotor and a stator that are magnetically coupled. Typically, the rotor has a coil that is used to produce a magnetic field. Electricity flowing through the rotor coil produces the magnetic field. In a generator, the rotor is coupled to a prime mover that rotates the rotor, producing a rotating magnetic field that induces a voltage in the stator. The voltage produced in the stator may be used to supply power to an electrical grid. In a motor, the stator produces a rotating magnetic field that interacts with the magnetic field produced by the rotor coil to induce rotation of the rotor.
Conventional copper conductors are commonly used to form the rotor coils. However, the electrical resistance of copper is sufficient to produce a substantial amount of resistive heat losses in the rotor coil, which reduces the efficiency of the rotating machine. In response to the losses caused by conventional copper conductors, superconductors have been developed for use as rotor coils. A superconductor is a material that loses its electrical resistance below a characteristic transition temperature, making it desirable for use as a rotor coil.
In rotating machines employing superconductive rotor coil, the rotor coil is cooled by a cryogenic fluid to lower the temperature of the superconductive coil below the transition temperature. Below the transition temperature, the superconductive rotor coil enters a superconducting state and loses its electrical resistance. Typically, a cryogenic fluid is provided to the superconductive coil by an external source of cryogenic fluid. The cryogenic fluid absorbs heat from the superconductive rotor coil, which maintains the rotor coil below the transition temperature and in a superconducting state. The cryogenic fluid for cooling the superconductive rotor coil is transferred between the source of cryogenic fluid and the rotor by a transfer coupling located at one end of the rotor shaft.
However, a number of problems are associated with transferring cryogenic fluid through the end of the rotor shaft. For example, a generator may be disposed between two turbines. In this arrangement, neither end of the rotor shaft is accessible to supply cryogenic fluid to the rotor coil.
There is a need, therefore, for an improved technique for transferring cryogenic fluid to a rotor of a superconductive rotating machine. There is a particular need for a technique, which can be employed to transfer cryogenic fluid to the rotor of a generator disposed between two prime movers, such as a gas turbine or a steam turbine.